Date of Award
Doctor of Philosophy (PhD)
Darnell James Laboratory
Signaling pathways involving Janus kinases (JAKs) and Signal Transducers and Activators of Transcription (STATs) play an important role in cytokine-induced gene transcription. Aberrant stimulation of this pathway can lead to abnormal development and cellular transformation demonstrating the importance of understanding mechanisms of downregulation. After interferon-γ (IFN-γ) treatment of cells the appearance of tyrosine phosphorylated Stat1 in the, nucleus was maximal within 20-30 minutes, remained for 2-2.5 hours and activated molecules disappeared by 4 hours. In the absence of continued signaling from the IFN-γ receptor (imposed by staurosporine treatment) previously activated Stat1 disappeared within 30-60 minutes, implying continuous generation and removal of tyrosine phosphorylated molecules during extended IFN -γ treatment. The proteasome, induced inhibitory proteins and the SHP-1 protein tyrosine phosphatase were all shown to play a role in the inactivation of the IFN -γ receptor-JAK complex but not in the direct inactivation of Stat1 molecules. By analyzing the flow of phosphorylated Stat1 from cytoplasm to nucleus and utilizing 35S labeling to examine the distribution of total Stat1 and activated Stat1 during a 4 hour IFN-γ time course, we concluded that the Stat1 molecules cycle into the nucleus as tyrosine phosphorylated molecules, are retained in the nucleus until dephosphorylated and then return to the cytoplasm. Our work with Stat5 activation by IL-2 suggests similar mechanisms of downregulation are involved in other JAK-STAT pathways. Therefore, the removal of the activated STAT molecules appears not to be proteolytic but must depend on a nuclear protein tyrosine phosphatase(s).
Haspel, Richard Lawrence, "Inactivation of the JAK-STAT Pathway" (1998). Student Theses and Dissertations. 449.